SET OTL ZNF Again

[xcortes]

As if I hadn't that many projects:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=160724631993#ht_3493wt_1877

Doc, We've talked about this before.

A 6c33 SET OTL ZNF would drive them no problem. Oh, and they seem to have a rising treble response so maybe, just maybe, they could be used with a 150hz front loaded horn which increases the tubes possibilities (how about a 6080).

Yes, I'm the buyer!

[xcortes]

How about a 300B? Unity factor would be just above one. Would an overdamped box correct this? Since the speaker has a rising HF response an underdamped amp would be a good match. Wouldn't it?

[Doc B.]

Might be better to use an OTL amp with an output impedance closer to 250-300 ohms. You could get the 300B lower than 800 ohms, but I'm not sure how much before output power would start to suffer. PJ may have played with this notion  abit during the 300B preamp design. They are supposed to be reasonably efficient and it might be interesting to start by driving them with a Crack amp and see what you get even though the power output is low.

[xcortes]

You beat me to it. Although I'm thinking building the Crack as a voltage amp, not a CF. A 6080 has a plate resistance of 280 ohms so no need to use CFs.

[xcortes]

I already have a semibuilt Crack!

[Paul Joppa]

You need to find the T-S parameters to answer the question of box design for a given source impedance. You may have to measure them yourself for that.

[xcortes]

Here's some measurements of other 9710 models. Of course the AM version will be different in the impedance related ones but maybe his can be used for a first approximation since besides the impedance of the voice coil they are the same drivers.

This guy did the measurements and some interesting modeling:

http://www.troelsgravesen.dk/philips9710.htm

According to him:

"Calculating a box for the 9710 isn't that easy as math dictates an excessively large cabinet. More than 100 litres closed for a Qtc of 0.7.
A 40 litres vented cabinet [] only provides a F3 of 56 Hz". So probably with an SET amp and an overdamped box I can lower than that? I can build the cabinets as large as required.

[Paul Joppa]

QE is pretty high there. With a 300 ohm power amp, it will be around 1.0, and very difficult to add enough mechanical damping to bring it down to ported box territory. I think an open baffle would be the best bet, followed by an "infinite baffle", i.e. a very large box, 200 liter or more. Both would still expose the xmax limitation, and a crossover to limit bass excursion (along with a sub or other deep bass augmentation) would probably help.

My two cents.

[xcortes]

Thanks Paul. Your two cents are worth two dollars for me.

So an OB it is. Simple to build. Maybe with a first order high pass filter at 120 hz or so. I will need a coupling cap at the output so I guess I could size it accordingly.

I take it'd rather use a 300 ohm plate tube than a 300B. A 6c33 would be great.

How about wiring a Crack as a voltage amp instead of as a cathode follower. How much power would I get driving the 800 ohm voice coil? It looks like maybe 750mW at 29mA, plate 70v and bias 10v?

Also, can the powe transformer of the Crack deliver more current? I guess so since you have a higher current version, right?

[Paul Joppa]

Yes, the Crack is set up as a FWB insead of a voltage doubler, so it can deliver twice the current of the SEX amp (same transformer). That is 140mADC total, both channels.

[Grainger49]

Paul,

I see questions about the Bottlehead power transformers fairly often.  And as you mention above the rectification scheme changes what is available from each one.

How about a sticky in Tech Topics that gives the voltage and current capability of each of the transformers?  That is if it isn't giving away too much.  There are folks out there renovating Bottlehead products to use them for crossovers or other amplifiers that need this information.

In the same sticky give the resulting voltage and current capability for the different rectifying schemes.  I'm always fuzzy on that.  Matter of fact that will help in a scratch build that Paul (Paully) and I are planning.

Just a suggestion....

[xcortes]

OK, I did some calaculations. Operating the Crack as a voltage amp it should provide around 750mW at 29mA, plate 70v and bias 10v. THD is pretty high at 16.7%

A higher current version at 70v plate, -6v bias and 65mA gives only 450mW but has a much better THD at 4.5%

The speakers are 91dB of efficiency. It would work as a test unless I horn load the Philips.

[Paul Joppa]

Get it up to 100v plate to cathode, at ~65mA. Should get 1.5 watts or so.

[xcortes]

Yes. 1.5 watts and 10% THD. Just below 500 ohm plate resistance. 12v bias.

 Cap coupled. unless I can run the 12AU7 at a different OP at around 55v plate voltage and still get enough drive. Would that work?

[Paul Joppa]

I'm getting 34v bias from the curves, at 100v P-K and 65mA. But an rp of 300 ohms. It's hard to read the Tung-Sol curves though.

You can run the 12AU7 with 1.2v bias or so, and at 2mA plate current it will have 50v at the plate with +/-25v peak output. Not enough to drive the above, but plenty for a 70v/45mA operating point.

You can easily have direct coupling, or a high-power operating point, or a C4S plate load; you can probably get any two of those with the Crack power supply, but not all three!